Stairs and ladders permitting ascent and descent between two levels have employed a wide variety of construction techniques to accomplish this well known function. Stairs are typically shipped as a relatively bulky pre-assembled unit or are fabricated and installed at the location of intended use. The need exists for lightweight stairs which may shipped in a knocked down or disassembled configuration to minimize shipping bulk and the associated cost of transport while permitting final assembly at the destination or use location. Moreover, it is desirable that such a stair permit ascent and descent at relatively steep angles of inclination to minimize use of valuable floor space. The minimization of floor space use is of particular significance aboard ships, vessels, and other locations where space is at a premium. Stairs and ladders known in the art are deficient in one or more of these aspects.
U.S. Pat. No. 3,110,132 discloses a pre-fabricated stair which provides for assembly of the stair without scaffolding at an intended use location. This stair includes a central supporting stringer and tread members which mateably engage the central stringer. The stair has parallel tread members and is not adapted to be disposed at steep angles of inclination.
U.S. Pat. No. 4,199,040 to the same inventor and assignee as the present invention discloses an alternating tread ladder adapted for use at relatively steep angles of inclination. This ladder is typically assembled at a place of manufacture and installed at a remote use location.
U.S. Pat. Nos. 4,061,202 and 4,069,892 disclose single stringer modular ladders which may be assembled and installed at a final use location.